United Airlines has apologised to a passenger who claims she had to sit with her toddler on her lap for the whole flight after the airline gave her child's seat away to a standby customer.

Shirley Yamauchi, 27, from Hawaii was making the three-and-a-half hour journey from Houston to Boston, the final leg of an 18-hour journey from Hawaii, on June 29. She claims she paid $1,000 USD for her son Taizo’s flight ticket, and was surprised to find a man claiming he had the same seat number as the two-year-old.

"I told him that I bought both of these tickets and he tells me that he got the ticket on standby. Then he proceeds to sit in the center,” Ms Yamauchi told Hawaii News Now.

Image: Shirley Yamauchi

She said she then mentioned the mix-up to a flight attendant, who told her that the plane was full and seemed uninterested.

"I had to move my son onto my lap. He's 25 pounds. He's half my height. I was very uncomfortable. My hand, my left arm was smashed up against the wall. I lost feeling in my legs and left arm," she said.

She also found out that the man in her son’s seat had paid just $75 for his ticket.

Yamauchi claims that she didn’t want to draw attention to the scene because she was afraid of the reaction.

"I started remembering all those incidents with United on the news. The violence. Teeth getting knocked out. I'm Asian. I'm scared and I felt uncomfortable. I didn't want those things to happen to me," she said.

The company faced backlash after footage of a passenger being dragged from a flight emerged.

The airline have since apologised to Yamauchi for the incident. A spokesman for United says because gate agents inaccurately scanned Taizo's boarding pass, their system showed him as not checked in, and his seat was released to another passenger.

The company said, "We deeply apologize to Ms. Yamauchi and her son for this experience. We are refunding her son's ticket and providing a travel voucher. We are also working with our gate staff to prevent this from happening again."